Differentiating exit() and exit_group() syscalls

Both exit() and exit_group() are tagged as Syscall.EXIT. Allong with this,
attribute for storing exit code parameter is introduced.
Parsing exit code argument of exit() and exit_group()

Both syscalls get as their argument exit code of the process that called
them. This has to be stored in separate attribute in parsed data so we can
easily decide on how the process terminated.